Dependence of Photocurrent Enhancements in Hybrid Quantum Dot

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Dependence of Photocurrent Enhancements in Hybrid Quantum Dot-MoS2 Devices on Quantum Dot Emission Wavelength John James Gough, Niall McEvoy, Maria O'Brien, John B. McManus, Jorge Alberto Garcia Coindreau, Alan P Bell, David McCloskey, Calin Hrelescu, Georg S. Duesberg, and A. Louise Bradley ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01681 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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Dependence of Photocurrent Enhancements in Hybrid Quantum Dot-MoS2 Devices on Quantum Dot Emission Wavelength John J. Gough1, Niall McEvoy2,3, Maria O’Brien2,3, John McManus2,3, Jorge Garcia-Coindreau1, Alan P. Bell2, David McCloskey1,3, Calin Hrelescu1, Georg S. Duesberg2,3,4, and A. Louise Bradley1 *. 1School

of Physics and CRANN, Trinity College Dublin, College Green, Dublin 2, Ireland

2School

of Chemistry and CRANN, Trinity College Dublin, College Green, Dublin 2, Ireland.

3AMBER

Centre, Trinity College Dublin, College Green, Dublin 2, Ireland

4Institute

of Physics, EIT 2, Faculty of Electrical Engineering and Information Technology,

Universität der Bundeswehr München, Werner-Heisenberg-Weg 39, 85577 Neubiberg, Germany TOC GRAPHIC

KEYWORDS: 2D materials, MoS2, quantum dots, photocurrent, energy transfer.

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ABSTRACT: The spectral dependence of nonradiative energy transfer (NRET) from three spectrally different quantum dot (QD) ensembles to monolayer MoS2 is reported. The QDs with peak emission wavelengths of 450 nm, 530 nm, and 630 nm induce large photocurrent enhancements in the hybrid devices with monolayer MoS2 islands grown by chemical vapor deposition (CVD). NRET efficiencies of over 90% are observed for each of the QD-MoS2 hybrids, with 3 fold to 6 fold photocurrent enhancements, depending on the spectral overlap between the QDs and the monolayer MoS2. We find good agreement between the trends obtained from the NRET rate and spectral overlap function showing evidence for a Förster-like energy transfer mechanism in these CdSeS/ZnS QD-MoS2 devices.

Research aimed towards the application of semiconducting 2D materials in optoelectronic technologies has gained substantial momentum in recent years.1,2 Graphene, in particular, has been the subject of intense study due to its extremely high mobility at room temperature (~10000 cm2/Vs)3 and fast photoresponse.4 However, the lack of a direct optical bandgap in graphene, due to its semi-metal character, and the high dark currents which arise due to these characteristics, limit its applicability in many optoelectronic devices.5 Transition metal dichalcogenides (TMDs) such as MoS2, on the other hand, possess the striking ability to transition from an indirect bandgap at few-layer thicknesses to a direct bandgap material at monolayer thicknesses.6–8 While sensitized photodetector devices consisting of graphene are known to display higher responsivity (~1 x108 A/W)9 than those consisting of TMDs such as MoS2 (1 x104 A/W)10, TMDs can be advantageous for applications that require a stronger light absorption11 and lower dark currents.12,13 TMDs are also of interest for electroluminescent light emitting devices.14


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Photodetectors with broadband optical absorption across the visible range are of interest for many applications. However, a significant drawback of TMDs is that the monolayer thickness limits the absorption of incoming light, and consequently, limits the overall device performance and efficiency. A straightforward and scalable method to overcome low absorption of incident light in these 2D materials is the addition of a sensitizing layer on top of the 2D material, most commonly organic dyes15 or quantum dots (QDs),13,16,17. However, several factors must be taken into account when selecting a sensitizing species including the sensitizer’s quantum yield, photostability and the spectral location of the optical absorption band. In terms of the aforementioned factors, QDs present many advantages over organic dye molecules including high quantum yields, improved photostability, broadband optical absorption and the ability to tune the emission profiles using the size or chemical composition dependence of the QDs.18–20 The broadband optical absorption associated with QDs is a particular advantage for light harvesting or photodetection devices operating across a larger optical bandwidth. A close proximity between the sensitizing species (donor) and the active material (acceptor) in the device allows for the optical energy to be transferred to the acceptor through nonradiative pathways.13,15,16 These nonradiative energy transfer pathways include charge transfer, at centre-tocentre distances between the donor and acceptor of 20 µm. The monolayer thickness of the devices was verified using Raman spectroscopy. Raman maps of devices D450, D530 and D630 are presented in Figure 2, panels d, e and f, respectively. These Raman maps plot the separation between the characteristic 𝐸′ and 𝐴′1 Raman peaks of the MoS2 Raman spectrum. Each of the Raman maps (Figure 2, panels d-f) show uniform regions in the device channels indicating that there is very


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little variation in the separation between the characteristic Raman peaks. Device D530, however has a large nucleation point in the central channel (not used for measurements) which can be clearly seen in the optical image (Figure 2b) and the Raman map (Figure 2e). Raman spectra were extracted from each of the Raman maps (averaged over the device channel area indicated by the white dotted box in Figure 2, panels d-f) to identify the layer number of the MoS2 islands. The Raman spectra obtained from each of the device channels reveal separations of ~18 cm-1 between the 𝐸′ and 𝐴′1 Raman peaks on each of the devices (See Figure S3 in the Supporting Information), verifying the monolayer thickness of the MoS2 in the devices.32,33 As mentioned above, previous reports in the literature regarding NRET from QDs to monolayer (and few-layer) MoS2 have utilized QDs with emission wavelengths located close to the MoS2 A and B excitons.28–31 However, a systematic spectral dependence has not been reported. The spectral dependence of NRET to monolayer MoS2 across the visible spectrum is a critical consideration for broadband applications and optimal spectral location of sensitizing species for use in a hybrid QD-MoS2 photodetector or light harvesting device.


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Figure 2: (a-c) Optical images of the devices. (d-f) Raman maps of the devices in (a-c) plotting the separation between the 𝐸′ and 𝐴′1 peaks in the Raman spectrum. The strength of the energy transfer between the QDs and the MoS2 devices was quantified using TRPL measurements. The PL decays were recorded over 3 µm x 3µm areas in the device channel (QD-MoS2) and 3 µm x 3µm areas close to the device containing no MoS2 (QDs). The PL decays presented in Figure 3 reveal the strong interaction between the QDs and the monolayer MoS2 through the significant decrease in the QD PL decay on the MoS2. The decrease in the QD lifetime is a characteristic signature of NRET. The PL decays were fitted with a bi-exponential fitting curve and the intensity weighted average lifetime was calculated using the fitting parameters (see section S4 in the Supporting Information for details). All lifetime values referred to in the text correspond to the average lifetime. The QD lifetime is reduced from ~20 ns to ~1 ns on each of the devices. The QD lifetime measured from a position close to the MoS2 devices containing no MoS2 is representative of the QD total spontaneous emission lifetime, 𝜏𝑄𝐷 = (𝑘𝑟 + 𝑘𝑛𝑟) ―1, where 𝑘𝑟 and 𝑘𝑛𝑟 are the radiative and nonradiative decay rates, respectively. The lifetime of the QDs on the MoS2 devices is 𝜏𝑄𝐷 ― 𝑀𝑜𝑆2 = (𝑘𝑟 + 𝑘𝑛𝑟 + 𝑘𝑁𝑅𝐸𝑇) ―1, where 𝑘𝑟 and 𝑘𝑛𝑟 are the same radiative and nonradiative decay rates as measured from the QDs off the MoS2 devices and 𝑘𝑁𝑅𝐸𝑇 is the rate of NRET from the QDs to the MoS2 devices. Therefore, the NRET rate can be calculated as 𝑘𝑁𝑅𝐸𝑇 =

(

)

―1 ―1 𝜏𝑄𝐷 ― 𝑀𝑜𝑆2 𝜏 , where 𝜏 𝜏𝑄𝐷 𝑄𝐷 ―𝑀𝑜𝑆2 𝑄𝐷 ― 𝑀𝑜𝑆2 ― 𝜏𝑄𝐷 and the NRET efficiency is then 𝜂𝑁𝑅𝐸𝑇 = 1 ―

and 𝜏𝑄𝐷 are the intensity weighted average lifetimes of the QDs on the monolayer MoS2 and the lifetime of the QDs alone, respectively. The NRET efficiencies are (94 ± 7)% for the 450 nm QDs on device D450, (92 ± 4)% for the 530 nm QDs on device D530, and (96 ± 6)% for the 630 nm QDs on device D630. Fluorescence lifetime maps were also recorded to further illustrate the strong


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quenching of the QD emission on the monolayer MoS2 triangles (see Figure S4 in Supporting Information). It should also be noted that in contrast to the highly efficient NRET process in this hybrid system, we observe no measurable enhancement of the MoS2 PL intensity, due to the nonradiative recombination of excitons at defect sites in the MoS2, as previously reported.17

Figure 3: PL decays of (a) 450 nm QDs alone (blue) and on MoS2 device D450 (black), (b) 530 nm QDs alone (green) and on MoS2 device D530 (black), and (c) 630 nm QDs alone (red) and on MoS2 device D630 (black). The dashed lines in each of the panels show the bi-exponential fits to the experimental data. Figure 4, panels a-c, show the photocurrent, 𝐼𝑃ℎ, measured from each device, with and without the QD sensitizing layers, as a function of optical excitation power at a voltage of +1 V, and the respective fits to the experimental data (dotted lines). It should be pointed out that an excitation wavelength of 405 nm was used for the TRPL measurements, photocurrent measurements, and the PL measurements used to calculate the spectral overlap. The open black data points in Figure 4, panels a-c, show the photocurrent obtained from the MoS2 devices before adding the QD sensitizing layers. The solid blue, green and red data points in Figure 4, panels a, b and c, are photocurrent values obtained after the addition of the 450 nm, 530 nm and 630 nm QD sensitizing layers, respectively. There is a clear enhancement of the generated photocurrent across the full


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excitation power range for each of the QD-MoS2 hybrid devices (Figure 4, panels a-c). The curves maintain similar dependences of photocurrent on the excitation power after the addition of the QD sensitizing layers. The photocurrent measured from each device exhibits a sub-linear dependence at low excitation powers which transitions to a linear dependence at mid-range powers and finally transitions to a super-linear dependence at the higher powers. While linear and sub-linear dependences of photocurrent on the optical excitation power are widely documented in the literature,34–38 super-linear dependences are less reported.39–41 This super-linear dependence of photocurrent on the optical excitation power has been explained by multi-centre recombination models.39 Given the monolayer thickness of the MoS2, a variety of recombination centers can be present due to imperfections including surface defects, edge states at grain boundaries and dangling bonds at the MoS2 surface, which could all contribute to the super-linear behavior.39,42– 45


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Figure 4: (a-c) Photocurrent measured from MoS2 devices as a function of laser excitation power before (open black triangles) and after the addition of the QD sensitizing layers (filled triangles). Dotted lines in a-c are fits to the experimental data. (d-f) Photocurrent enhancement of MoS2, 𝐸𝐴𝑐𝑐, due to the presence of the QD sensitizing layers as a function of laser excitation power. (g-i) Photoresponsivity as a function of optical excitation power for each of the MoS2 devices before (open black squares) and after the addition of the QD sensitizing layers (450 nm QDs blue squares in g, 530 nm QDs green squares in h, 630 nm QDs red squares in i).


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A polynomial of the form 0.5

𝐼𝑃ℎ = 𝐴𝑃𝑒𝑥 + 𝐵𝑃𝐸𝑥 + 𝐶𝑃2𝐸𝑥

(1)

is found to fit photocurrent data measured from the devices before and after adding the QD sensitizing layers, as can be seen in Figure 4, panels a-c. 𝐴, 𝐵, and 𝐶 are fitting coefficients corresponding to sub-linear, linear and super-linear regimes, respectively. The coefficients extracted from the fits using eqn (1) are presented in Figure 4, panels a-c. To quantify the average enhancement of photocurrent across the full excitation power range in the MoS2 devices due to the inclusion of the QD sensitizing layers, the average photocurrent ratio, 〈𝐸〉, was considered

〈𝐼𝑃ℎ 𝑀𝑜𝑆 ― 𝑄𝐷〉 〈𝐸〉 = 〈𝐼𝑃ℎ 𝑀𝑜𝑆 〉 2

(2)

2

where 〈𝐼𝑃ℎ 𝑀𝑜𝑆2 ― 𝑄𝐷〉 is the average photocurrent measured from the QD-sensitized devices and

〈𝐼𝑃ℎ 𝑀𝑜𝑆 〉 is the average photocurrent measured from the same devices without the QD sensitizing 2

layer. The average is taken over the full excitation power range. The average ratio, 〈𝐸〉, is 4 ± 1, 4 ± 1 and 7 ± 2 for device D450, D530 and D630, respectively. Similar to the NRET efficiencies measured from the QD lifetimes on each of the devices, we find the largest average increase on device, D630 and the lowest average increase on device D530. As was mentioned earlier, the 450 nm QDs are close to resonance with the MoS2 C exciton and the 630 nm QDs are on-resonance with the MoS2 B exciton, while the 530 nm QDs provide an offresonance reference (Figure 1a). The NRET efficiency and rate depends strongly on the spectral overlap between the QD PL spectrum and the MoS2 absorption spectrum, and it will be shown later that the spectral dependence of the photocurrent displays the same trend as the NRET efficiency.


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The photocurrent enhancement in the MoS2 energy acceptor due to the NRET from the QDs, is defined as 𝐸𝐴𝑐𝑐 =

𝑃ℎ 𝐼𝑃ℎ 𝑀𝑜𝑆2 ― 𝑄𝐷 ― 𝐼𝑀𝑜𝑆2

𝐼𝑃ℎ 𝑀𝑜𝑆2

(3)

𝑃ℎ where 𝐼𝑃ℎ 𝑀𝑜𝑆2 ― 𝑄𝐷 and 𝐼𝑀𝑜𝑆2 are the photocurrent values of the QD-MoS2 (donor-acceptor) hybrids

and the MoS2 only (acceptor) devices, respectively. For each of the QD-MoS2 hybrid devices in Figure 4, panels d-f, there is a step-like increase in the photocurrent enhancement, 𝐸𝐴𝑐𝑐, as the excitation power increases, while the enhancement appears to saturate and remain constant at higher excitation powers. In terms of the optimal excitation power range, the largest photocurrent enhancements are found at the higher excitation powers in the super-linear regime. The 630 nm QD sensitizing layer gives the largest acceptor enhancement, of 7.0 ± 0.7 at the maximum excitation power of ~140 µW. The 450 nm and 530 nm QD sensitizing layers lead to acceptor enhancements of 4.4 ± 0.4 and 5.0 ± 0.5, respectively, at the same excitation power. The fact that the 630 nm QDs give the largest enhancement of the MoS2 photocurrent is beneficial also in terms of maximizing the absorption of white light as the 630 nm QDs absorb over a larger spectral region of the visible spectrum compared to the 530 nm and 450 nm QDs (Figure 1a). The photoresponsivity, 𝑅, is a measure of a device’s output current as a function of optical excitation power. 𝑅 = 𝐼𝑃ℎ/𝑃𝑒𝑥, where 𝑃𝑒𝑥 is the optical excitation power. The photoresponsivity curves for the hybrid 450 nm, 530 nm and 630 nm QD-MoS2 devices are presented in Figure 4, panels g, h and i, respectively. Each curve (with and without the QD sensitizing layer) shows a decreasing trend in the photoresponsivity as the excitation power increases at low powers (~1-10 µW). This decrease in the photoresponsivity corresponds to a sub-linear dependence of photocurrent on the optical excitation power. When the optical excitation power reaches tens of


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µWs (depending on the device), the photoresponsivity begins to level off and remains constant over a small intermediate power range. This constant photoresponsivity is the result of a linear dependence of the generated photocurrent, 𝐼𝑃ℎ, on the optical excitation power, 𝑃𝑒𝑥. With a further increase in the optical excitation power, the photoresponsivity begins to rise, as a result of the super-linear dependence of generated photocurrent on the optical excitation power. This behavior is observed for each of the devices presented in this study, both with and without the QD sensitizing layers.

Figure 5: Looping I-V curves for each of the devices before adding the QDs (black) and after adding the (a) 450 nm QDs (blue), (b) 530 nm QDs (green), and (c) 630 nm QDs (red). The measured current-voltage (I-V) characteristics of the MoS2 and the hybrid QD-MoS2 devices are presented in Figure 5. These I-V curves were measured at an excitation power of 100 μW. There is a clear enhancement in the photocurrent after the addition of the QD-sensitizing layers, however, there is some hysteresis in the curves as they sweep back from the maximum voltage. The sweep out from 0 V to +1 V (-1 V) has a close to linear form while the sweep back from the maximum voltage demonstrates a drop in current, as indicated by the arrows (Figure 5). This hysteresis effect is well documented and is attributed to charge trapping at surface defect states at the interface between the MoS2 and the surrounding medium.46 This evidence of the


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presence of surface trap states further supports the hypothesis that it is the presence of these defects that contribute to the super-linear behaviour of the photocurrent.39 As discussed earlier, the rate and efficiency of Förster-type NRET between donor-acceptor pairs is governed by the centre-to-centre separation, the donor quantum yield, and the spectral overlap, 𝐽, between the donor emission and the acceptor absorption spectra. Given that each of the QD samples used in this study has the same diameter (6.0 ± 0.8) nm, the centre-to-centre distance is the same for each hybrid device. It should also be noted that the three QDs also have the same quantum yield, (50 ± 5)%. The Förster NRET rate between a donor and acceptor dipole pair at a given centre-to-centre separation, 𝑑, is given by22 1 𝑅0 𝑘𝑁𝑅𝐸𝑇 = 𝜏𝐷 𝑑

6

( )

(4)

where 𝜏𝐷 is the donor lifetime and 𝑅0 is the Förster radius, the characteristic distance at which the NRET efficiency is 50%. It is expressed by

𝑅0 =

(

2

)

9000 ∙ 𝑙𝑛 (10) ∙ 𝜅 ∙ 𝑄𝑌 ∙ 𝐽 128 ∙ 𝑁𝐴 ∙ 𝜋5 ∙ 𝑛4

1 6

(5)

where 𝜅2 is a factor that describes the relative orientation between donor and acceptor dipoles, 𝑄𝑌 is the donor quantum yield, 𝐽 is the spectral overlap between the donor emission spectrum and the acceptor absorption spectrum, and 𝑛 is the refractive index. Filling eqn (5) into eqn (4) highlights the proportionality between the NRET rate and the spectral overlap

(

𝑘𝑁𝑅𝐸𝑇 = 𝐽𝜏𝐷―1

)(

𝑄𝑌 ∙ 𝜅2 6

𝑑

9000 ∙ 𝑙𝑛 (10)

)

128 ∙ 𝑁𝐴 ∙ 𝜋5 ∙ 𝑛4

(6)

The spectral overlap integral, 𝐽, is given by


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∞

𝐽=

∫𝐼

𝑄𝐷(𝜆)

∙ 𝜀𝑀𝑜𝑆2(𝜆) ∙ 𝜆4 𝑑𝜆

(7)

0

where 𝐼𝑄𝐷(𝜆) is the area normalised donor emission spectrum and 𝜀𝑀𝑜𝑆2(𝜆) is the acceptor extinction coefficient. As the MoS2 islands used in this study are single flakes on a solid substrate, an accurate extinction coefficient could not be obtained. However, in terms of the spectral overlap calculation, the extinction coefficient, 𝜀𝑀𝑜𝑆2, only differs from the absorption, 𝛼𝑀𝑜𝑆2, by a scaling factor. The absorption coefficient can be measured directly from our samples. Therefore, in order to obtain a qualitative comparison of 𝑘𝑁𝑅𝐸𝑇 predicted by eqn (6) and that given by the ―1 ―1 experimentally measured PL lifetimes, 𝑘𝑁𝑅𝐸𝑇 = 𝜏𝑄𝐷 ― 𝑀𝑜𝑆2 ― 𝜏𝑄𝐷 , the extinction coefficient can

be replaced by the absorption coefficient in eqn (7). As the QD-MoS2 separation, 𝑑, and the quantum yield are the same for each of the QDs, the plot of 𝜏𝐷―1 ∙ 𝐽 is compared with 𝑘𝑁𝑅𝐸𝑇 = ―1 ―1 𝜏𝑄𝐷 ― 𝑀𝑜𝑆2 ― 𝜏𝑄𝐷 in Figure 6. There is clear agreement between the two trends of the NRET rate

obtained from the lifetime data and that determined by spectral overlap. This is evidence that the energy transfer process in this hybrid system is Förster-type NRET.

Figure 6: Plot of the NRET rate, 𝑘𝑁𝑅𝐸𝑇, and the product of the inverse donor lifetime, 𝜏𝐷―1, and the spectral overlap, 𝐽, as a function of the QD emission wavelength.


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It can also be noted that the QD dependence of the NRET rate shows the same trend as the photocurrent enhancement. The NRET rate is calculated using lifetime data recorded at low excitation power, and therefore, for comparison the average photocurrent enhancement, 𝐸𝐴𝑣𝑔 𝐴𝑐𝑐 , is 𝐴𝑣𝑔 calculated using the A coefficients from eqn(1) 𝐸𝐴𝑐𝑐 = (𝐴𝑀𝑜𝑆2 ― 𝑄𝐷 𝐴𝑀𝑜𝑆2) ― 1 and shown in

Figure 7. To explore further how the photocurrent enhancement can be expected to depend on the Förstertype NRET we consider that in a system with an emitting acceptor the NRET efficiency can be expressed in terms of the acceptor emission enhancement47 𝜂𝑁𝑅𝐸𝑇 =

(

𝐴𝑏𝑠𝐴𝑐𝑐 𝐼𝐸𝑚 𝐴𝑐𝑐 ― 𝐷𝑜𝑛 𝐴𝑏𝑠𝐷𝑜𝑛

𝐼𝐸𝑚 𝐴𝑐𝑐

)

―1

(8)

where 𝐴𝑏𝑠𝐴𝑐𝑐 𝐴𝑏𝑠𝐷𝑜𝑛 is the acceptor:donor absorption ratio (constant) at the excitation wavelength, 𝐼𝐸𝑚 𝐴𝑐𝑐 ― 𝐷𝑜𝑛 is the integrated emission intensity of the acceptor in the presence of the donor, 𝐼𝐸𝑚 𝐴𝑐𝑐 is the integrated emission intensity of the acceptor in the absence of the donor. As mentioned earlier, we observe no measurable enhancement in the MoS2 PL intensity despite observing highly efficient NRET from the QDs. This lack of PL enhancement is attributed to the nonradiative recombination at defects in the MoS2.17 However, by replacing the intensity with photocurrent in eqn (8), the relationship between the photocurrent enhancement and NRET efficiency would become 𝜂𝑁𝑅𝐸𝑇 =

𝐴𝑏𝑠𝑀𝑜𝑆2 𝐸𝐴𝑣𝑔 𝐴𝑏𝑠𝑄𝐷 𝐴𝑐𝑐

(9)

where 𝐸𝐴𝑣𝑔 𝐴𝑐𝑐 is the average photocurrent enhancement. Therefore, the photocurrent enhancement could be expected to scale with the NRET efficiency. The experimental QD dependence of the


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NRET efficiency, calculated from the PL lifetime data, and the low power average photocurrent enhancement are shown in Figure S6 in the SI. While they are seen to have similar trends, as expected from eqn(9), unfortunately the errors on the efficiencies are too large to be conclusive.

Figure 7: The low power average photocurrent enhancement, 𝐸𝐴𝑣𝑔 𝐴𝑐𝑐 , as a function of the QD emission wavelength.

CONCLUSIONS In summary, ultra-high efficiency NRET from three spectrally separated QD ensembles to monolayer MoS2 has been demonstrated. The emission spectra for the QD ensembles span the visible region and fulfil conditions of being on- and off the MoS2 exciton absorption peaks. Timeresolved photoluminescence measurements reveal NRET efficiencies exceeding 90% for each QD ensemble, with values of (94 ± 7)%, (92 ± 4)% and (96 ± 6)% for the 450 nm, 530 nm, and 630 nm QDs, respectively. Photocurrent measurements were performed on the devices before and after


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adding the QD-sensitizing layers. Average photocurrent ratios of 4 ± 1, 4 ± 1 and 7 ± 2 were measured for the D450, D530 and D630 devices, respectively. The average low excitation power photocurrent enhancement is 2.9 ± 0.5, 1.5 ± 0.1 and 5.9 ± 1.1, while the largest enhancement of the MoS2 photocurrent is found at the highest excitation power (~140 µW), with values of 4.4 ± 0.4, 5.0 ± 0.5 and 7.0 ± 0.7 for devices D450, D530, and D630, respectively. We observe the emergence of a super-linear dependence of photocurrent at high excitation power for each of the MoS2 devices, with and without the QD-sensitizing layers. This is attributed to multi-centre recombination due to the presence of surface defects. As a consequence of the super-linear behavior, the photoresponsivity at higher powers recovers and approaches values close to those measured at lower excitation powers. Comparison of the trends in the NRET rates and the spectral overlap suggests that the energy transfer mechanism in the hybrid system is Förster-type NRET. Regarding the significance of the spectral position of the QDs, we find the largest enhancement for the hybrid 630 nm QD device. This is beneficial also in terms of a solar harvesting system as the optical absorption of the 630 nm QDs has greater spectral coverage as compared to the 450 nm and 530 nm QDs.

MATERIALS AND METHODS The growth of triangular islands of monolayer MoS2 was achieved using a previously reported chemical vapor deposition (CVD) growth technique.48 Briefly, a microreactor was formed between the seed and target substrates whereby the target substrate is placed face down on top of a seed substrate, which consists of liquid-phase exfoliated MoO3 nanosheets drop cast onto a Si/SiO2 substrate.49 This microreactor was then placed into the center of a quartz tube furnace where it was


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heated to 750 oC under 150 sccm Ar flow and then exposed to sulfur vapor, which was generated by heating sulfur powder to ~120 oC in an independently-controlled upstream zone of the furnace. Alloyed CdSeS/ZnS QDs (1 mg/mL) in toluene with peak emission wavelengths of 450 nm, 530 nm and 630 nm, were purchased from Sigma-Aldrich. The QDs were used as supplied in stock solution. The QD/Poly(methyl methacrylate) (PMMA) solutions were prepared by dispersing 12.5 µL of each stock solution of QDs in 500 µL of 0.1% wt. PMMA in toluene. The QD/PMMA solutions were sonicated for ~20 s to ensure even dispersion of the QDs in the PMMA. The 450 nm, 530 nm and 630 nm QDs have diameters of (6.0 ± 0.8) nm, see Figure S1 in the Supporting Information. The extinction spectrum of the monolayer MoS2 was measured from a CVD grown triangular MoS2 monolayer flake which had been grown on a quartz substrate. The extinction measurement was performed using a custom built transmission apparatus consisting of a Xenon lamp and a 100x microscope objective. The CVD-grown MoS2 used in this study was characterized by Raman spectroscopy. The Raman measurements were performed using a WITec Alpha 300R tool with a 532 nm excitation laser operating at a power of ~250 µW with a 100x objective (NA = 0.95). The Raman maps were obtained via the acquisition of 4 spectra per µm in x and y directions. Time-resolved photoluminescence (TRPL) measurements were carried out using a PicoQuant Microtime200 time-resolved confocal microscope system with optical excitation at a wavelength of 405 nm by pulses with full-width at half-maximum of 150 ps at a repetition rate of 10 MHz. The integration time was 4 ms per pixel. The laser spot size was ~430 nm. The samples were excited through a 40x objective (NA = 0.65) and the PL was collected through the same objective. The excitation power used for the TRPL measurements was 0.2 µW.


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All scans were performed over 3 µm x 3 µm square areas in the device channels (QD only measurements were performed over 3 µm x 3 µm square areas beside the devices where there was no MoS2). Bandpass filters, centered at 450 nm with a 40 nm full-width-at-half-maximum (FWHM) for the 450 nm QDs, 500 nm with a 40 nm FWHM for the 530 QDs, and 635 nm with a 10 nm FWHM for the 630 nm QDs, were used to ensure that only the PL from the QDs was collected. The electrical devices were fabricated by patterning a PMMA resist using electron beam lithography (EBL). The contact pads and electrodes consist of Ti/Au (5 nm/ 45 nm) and were deposited using electron beam evaporation. The contact pad dimensions are 80 µm x 80 µm and each device had a 5 µm channel width between the electrodes. The MoS2 devices discussed in this manuscript were grown on separate Si/SiO2 chips in a single growth process. The hybrid devices were fabricated by spin casting an ultra-thin layer (~7-9 nm, see Figure S5 in the Supporting Information) of dilute QDs in 0.1% wt. PMMA (12.5 µL QDs in 500 µL PMMA) to achieve monolayer coverage of QDs on the MoS2 devices. The optical excitation at 405 nm for the photocurrent measurements was achieved using a variable power Toptica iBeam smart laser diode. The device channels were excited through a 10x (NA = 0.25) objective resulting in an excitation spot diameter of ~3µm. The electrical measurements were performed using a Keithley 2400 source meter to provide a bias and simultaneously measure the current through the devices. ASSOCIATED CONTENT Supporting Information. Additional TEM, AFM, and optical characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Funding Sources This work was supported by Science Foundation Ireland (SFI) under grant numbers 16/IA/4550, 10/IN.1/12975, 15/IA/3131 and 15/SIRG/3329. JJG acknowledges a postgraduate research scholarship from the Irish Research Council (IRC) GOIPG/2013/680. MO’B acknowledges a postgraduate research scholarship from the Irish Research Council (IRC) via the Enterprise Partnership Scheme, Project 201517, Award number 12508. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge Robert O’Connell for performing the AFM measurements of the QD/PMMA layers, Eoin McCarthy for the TEM measurements and Lisanne Peters for assistance with device fabrication

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Figure 1: (a) Normalized extinction spectrum of monolayer MoS2 (black), and scaled extinction spectra of the 450 nm QDs (blue dot), 530 nm QDs (green dot) and 630 nm QDs (red dot), and normalized PL spectra of 450 nm QDs (blue), 530 nm QDs (green) and 630 nm QDs (red). The QD extinction spectra are scaled to have the first absorption peak at the same intensity, for presentation purposes. (b) Schematic diagram of experimental devices. 302x126mm (150 x 150 DPI)


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Figure 2: (a-c) Optical images of the devices. (d-f) Raman maps of the devices in (a-c) plotting the separation between the E^' and A_1^' peaks in the Raman spectrum. 323x172mm (150 x 150 DPI)


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Figure 3: PL decays of (a) 450 nm QDs alone (blue) and on MoS2 device D450 (black), (b) 530 nm QDs alone (green) and on MoS2 device D530 (black), and (c) 630 nm QDs alone (red) and on MoS2 device D630 (black). The dashed lines in each of the panels show the bi-exponential fits to the experimental data. 312x100mm (150 x 150 DPI)


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Figure 4: (a-c) Photocurrent measured from MoS2 devices as a function of laser excitation power before (open black triangles) and after the addition of the QD sensitizing layers (filled triangles). Dotted lines in a-c are fits to the experimental data. (d-f) Photocurrent enhancement of MoS2, E_Acc, due to the presence of the QD sensitizing layers as a function of laser excitation power. (g-i) Photoresponsivity as a function of optical excitation power for each of the MoS2 devices before (open black squares) and after the addition of the QD sensitizing layers (450 nm QDs blue squares in g, 530 nm QDs green squares in h, 630 nm QDs red squares in i). 308x264mm (150 x 150 DPI)


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Figure 5: Looping I-V curves for each of the devices before adding the QDs (black) and after adding the (a) 450 nm QDs (blue), (b) 530 nm QDs (green), and (c) 630 nm QDs (red). 422x147mm (150 x 150 DPI)


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Figure 6: Plot of the NRET rate, k_NRET, and the product of the inverse donor lifetime, τ_D^(-1), and the spectral overlap, J, as a function of the QD emission wavelength. 163x117mm (150 x 150 DPI)


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Figure 7: The low power average photocurrent enhancement, E_Acc^Avg, as a function of the QD emission wavelength. 163x117mm (150 x 150 DPI)


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TOC Graphic 71x40mm (150 x 150 DPI)


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Dependence of Photocurrent Enhancements in Hybrid Quantum Dot

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